1. Field of the Invention
The present invention relates to a heat exchanger tube, a method of manufacturing a heat exchanger tube, and a fluidized-bed furnace.
2. Description of the Related Art
A fluidized-bed furnace for incineration disposal of waste includes a fluidized layer which is made of fluidized sand such as silica sand deposited on a furnace bed. The fluidized-bed furnace performs heating by blowing up the fluidized sand while jetting fluidizing gas such as air into the fluidized layer from the furnace bed, and a material to be incinerated such as waste is stirred while being mixed with the fluidized sand at a high temperature, whereby the material to be incinerated is dried, thermally decomposed, and incinerated.
In a furnace casing of such a fluidized-bed furnace, a heat exchanger tube (cooling tube) is provided to adjust the temperature of the fluidized sand and the material to be incinerated (see, Japanese Patent Application Laid-open No. 2004-93058 and Japanese Patent Application Laid-open No. 2005-315566). A refrigerant such as water is made to flow through the inside of the heat exchanger tube, and the fluidized sand and the material to be incinerated which come into contact with a surface of the heat exchanger tube are cooled by being heat-exchanged with the refrigerant in the heat exchanger tube via inner and outer surfaces of the heat exchanger tube.
Generally, materials of heat exchanger tubes used in various apparatuses are selected in consideration of heat resistance against temperatures of atmospheres in which the heat exchanger tubes are used. For example, carbon steel is mainly used in an atmosphere of 350° C. or lower, a low-alloy steel in an atmosphere of 500° C. or lower, a chromium-containing steel (Cr: 9% to 12%) in an atmosphere of 550° C. or lower, and austenitic stainless steel in an atmosphere of 550° C. or higher.
However, the temperature to which the heat exchanger tube is exposed in the fluidized-bed furnace ranges from about 500° C. to about 1100° C., and in addition, in the fluidized-bed furnace, highly corrosive gas containing HCl (hydrogen chloride), Cl2 (chlorine), or the like is generated by the incineration disposal of a material to be incinerated. Further, around the heat exchanger tube, the fluidized sand and the material to be incinerated are actively fluidized. In particular, sharp and hard substances such as incombustible metal pieces and sands are mixed in the material to be incinerated, and these substances incessantly collide with the heat exchanger tube. Conventionally, it has been difficult for a heat exchanger tube to exhibit sufficiently high performance even under such an environment and it has not been possible to satisfy all of heat resistance against high temperature in the fluidized-bed furnace, abrasion resistance against the contact of the fluidized sand and the material to be incinerated, strength against pressure generated by the blowing of the fluidizing gas, corrosion resistance against the corrosive gas, ductility which prevents cracking even if the heat exchanger tube expands and contracts in accordance with a temperature change, heat conductivity which enables efficient adjustment of the temperature of the fluidized sand and the material to be incinerated, and so on. Consequently, deterioration such as abrasion, damage, corrosion, and cracking easily occurred in the heat exchanger tube and thus it has been difficult to prolong the life of the heat exchanger tube, which consequently has necessitated frequent inspection, repairing, replacement, and the like of the heat exchanger tube. This has made it difficult to reduce trouble and cost required for such works.